1. Field of the Invention
The present invention relates in general to an economical and efficient process for removing fluoride from wastewater.
2. Description of the Related Art
In various industries, such as the production of semiconductors, chlorofluorocarbon (CFC) and glass, a large amount of fluoride-containing wastewater with a high concentration of fluoride is produced. Therefore, many researchers have attempted to remove fluoride from the fluoride-containing wastewater.
Jansen in U.S. Pat. No. 5,106,509 has disclosed a crystallization process for removing fluoride from wastewater in a fluidized bed reactor. The process involves adding CaCl.sub.2 into wastewater to react calcium ions and fluoride in the wastewater to form calcium fluoride crystals. Compared with the coagulation/precipitation process that is used in factories to date, the advantages of Jansen's process are that the waste sludge is decreased and can be recycled. Since calcium fluoride has a very low solubility, such a crystallization process can effectively remove a great amount of fluoride. However, another result of the low solubility is that calcium fluoride will easily supersaturate in some locations, thus generating fine particles that may clog the pipes. For this reason, in practical use, wastewater discharged from the factory with a high concentration of fluoride should be diluted to a concentration lower than 500 mgF.sup.- /l. In order to accommodate such a great amount of diluted wastewater, the cost of manufacturing the apparatus for processing wastewater and the space occupied by the apparatus increase.
Japanese Patent No. 6-88026 has disclosed another process for removing fluoride from wastewater, in which sodium, calcium, aluminum and fluoride ions are maintained at appropriate ratios to form pachnolite. Then, the pachnolite is filtered out and recycled. Japanese Patent No. 5-15882 has disclosed another process for removing fluoride from wastewater, in which sodium, calcium and aluminum reagents are mixed and then added into the wastewater to form pachnolite. The two Japanese Patents are not so successful because the effectiveness of the removal fluoride is not very good (by the former, the fluoride concentration is reduced from 1,000 mgF.sup.- /l to 100 mgF.sup.- /l, and by the latter, the fluoride concentration is reduced from 2,000 mgF.sup.- /l to 116 mgF.sup.- /l) and the required reaction time is long (3.5 hours). In addition, the obtained product (pachnolite) is in form of particles. Separating the particles from the treated wastewater and transporting the particles are not easy.